About thirty years ago it became apparent that household laundry detergents made of heavily branched alkylbenzene sulfonates were gradually polluting rivers and lakes. Solution of the problem led to the manufacture of detergents made of linear alkylbenzene sulfonates (LABS) and later modified linear alkylbenzene sulfonates (MLABS), both of which were found to biodegrade more rapidly than the heavily branched variety. Today, detergents made using LABS and MLABS are known.
LABS are manufactured from linear alkylbenzenes (LAB), and MLABS can be made from modified linear alkylbenzenes (MLAB). The petrochemical industry produces LAB by dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of HF. This is the industry's standard process.
Over the last decade, environmental concerns over HF have increased, leading to a search for substitute processes employing catalysts other than HF that are equivalent or superior to the standard process. Solid alkylation catalysis to produce LAB, for example, is the subject of vigorous, ongoing research. Solid alkylation catalysts can also be used to produce MLAB and are also being researched vigorously. It is known that MLAB may be made by dehydrogenating slightly branched paraffins to slightly branched olefins and then alkylating benzene with the slightly branched olefins in the presence of a solid catalyst. See, for example, U.S. Pat. Nos. 6,111,158 B1 and 6,187,981 B1, which are incorporated herein by reference.
As desirable as solid catalyst may be as an alternative to liquid HF, it is commonly the case that these catalysts deactivate with use. All alkylation catalysts, including HF and substitute catalysts for HF, lose some portion of their activity with continued use. However, the solid catalysts used to date in aromatic alkylation tend to deactivate rather quickly. Solid catalysts used for alkylation of aromatic compounds by olefins, especially those in the 6 to 22 carbon atom range, usually are deactivated by gum-type materials that accumulate on the surface of the catalyst and block reaction sites. These materials include byproducts, such as aromatic (including polynuclear) hydrocarbons in the 10 to 22 carbon atom range, that are formed in the dehydrogenation of C6 to C22 paraffins. These materials also include undesired alkylation byproducts of higher molecular weight than the desired monoalkyl benzenes, e.g., di- and tri-alkyl benzenes, as well as olefin oligomers and other olefinic compounds.
An alkylation process using a solid alkylation catalyst typically includes means for periodically taking the catalyst out of service and regenerating it by removing these deactivating materials from the catalyst. For a solid alkylation catalyst, the catalyst life is measured in terms of time in service at constant conversion between regenerations. The longer the time between regenerations, the more desirable the catalyst and the process. Thus, it is clear that solid catalyst can be best used in the continuous alkylation of aromatics only where effective and inexpensive means of catalyst regeneration are available. Fortunately it has been observed that the deactivating materials can be readily desorbed from the catalyst by washing the catalyst with the aromatic reactant (e.g., benzene). Thus, catalyst reactivation, or catalyst regeneration as the term is more commonly employed, is conveniently effected by flushing the catalyst with an aromatic such as benzene to remove the accumulated deactivating materials from the catalyst surface, generally with restoration of 100% of catalyst activity.
A typical prior art means for regenerating the solid catalyst in an aromatic alkylation process is described in U.S. Pat. No. 6,069,285. The effluent of an alkylation reactor undergoing regeneration combines with the effluent of an on-stream alkylation reactor, and the combined effluent passes to a section of the process for recovering benzene, the alkylated benzene product, and other streams. In U.S. Pat. No. 6,069,285, this section comprises a benzene rectifier, a benzene fractionation column, and other product recovery facilities. Part of the benzene recovered from this section is recycled to the off-stream alkylation reactor to regenerate the deactivated catalyst. Another prior art process passes the effluent of the reactor undergoing regeneration to a separation zone to reject color bodies and to recover benzene that passes to the benzene fractionation column of this section.
Besides regeneration, another means for maintaining high catalyst activity is to prevent the previously mentioned aromatic byproducts formed in the dehydrogenation of paraffins from ever entering the alkylation reactors. These aromatic byproducts are believed to include, for example, alkylated benzenes, naphthalenes, other polynuclear aromatics, alkylated polynuclear hydrocarbons in the C10–C15 range, indanes, and tetralins, that is, they are aromatics of the same carbon number as the paraffin being dehydrogenated and may be viewed as aromatized normal paraffins. They are typically removed using an aromatics removal zone, such as those described in U.S. Pat. Nos. 5,276,231; 5,334,793; and 6,069,285, the contents of which are incorporated herein by reference. Fixed bed sorptive separation zones that use a particulate sorbent, such as a molecular sieve (e.g., 13 X zeolite (sodium zeolite X)), are the most common aromatics removal zones.
In a typical fixed bed system, the sorbent is installed in two or more vessels in a parallel flow arrangement, so that when the sorbent bed in one vessel is spent by the accumulation of the aromatic byproducts thereon, the spent vessel is bypassed while continuing uninterrupted operation through another vessel. A purge stream comprising a purge component, such as C5 or C6 paraffin (e.g., normal pentane), is passed through the spent sorbent bed in the bypassed vessel in order to purge or displace unsorbed components of the stream containing the aromatic byproducts from the void volume between particles of sorbent. After purging, a regenerant or desorbent stream comprising a desorbent component such as C6 or C7 aromatic (e.g., benzene), is passed through the sorbent bed in the bypassed vessel in order to desorb aromatic byproducts from the sorbent. Following regeneration, the sorbent bed in the bypassed vessel is again available for use in sorbing aromatic byproducts.
Thus, a sorptive separation zone for removing the aromatic byproducts typically produces three effluents, which approximately correspond to each of the three steps in the cycle of sorption, purge, and desorption. The composition of each of the three effluents can change during the course of each step. The first effluent, the sorption effluent, contains unsorbed components (i.e., paraffins and olefins) of the stream from which the aromatic byproducts are removed, and also typically contains the desorbent component. With its decreased amount of aromatic byproducts relative to the stream that is passed to the sorptive separation zone, this effluent is used further along in the process to produce alkylaromatics. For example, if the stream that passes to the sorptive separation zone is the dehydrogenation zone effluent, the sorption effluent contains monoolefins and paraffins and thus passes directly to the alkylation zone.
The second effluent, the purging effluent, contains the purge component, unsorbed components of the stream from which the aromatic byproducts were sorbed, and often the desorbent component. The third effluent is the desorption effluent, which contains the desorbent component, the aromatic byproducts, and the purge component. The purging and desorption effluents typically are separated in two distillation columns. The desorption effluent passes to one column, which produces an overhead stream containing the desorbent and purge components and a bottom stream containing the aromatic byproducts which is rejected from the process. The overhead stream of the first column and the purging effluent pass to a second column, which separates the entering hydrocarbons into an overhead stream containing the purge component and a bottom stream containing the desorbent component and unsorbed components of the stream from which the aromatic byproducts are removed. The overhead stream of the second column is used as the purge stream. The bottom stream of the second column is used in the process to produce alkylaromatics. In the example described above where the stream that passes to the sorptive separation zone is the dehydrogenation zone effluent, the bottom stream of the second column contains benzene, monoolefins, and paraffins and flows directly to the alkylation zone. Some of the benzene in this bottom stream passes through the alkylation reactor unreacted, and is recovered in the previously mentioned section for separating the alkylation reactor effluent.
Unfortunately, the prior art uses benzene inefficiently. Separating, recovering, and recycling benzene for the on-stream alkylation reactor is a huge cost by itself. But the prior art requires much more than just that amount of benzene, because of the benzene for catalyst regeneration and/or sorbent desorption. The rewards of large reductions in capital investment and operating expenses are the incentive to developing new ways to use benzene more efficiently in aromatic alkylation processes.